Many solid materials are industrially recrystallized from solution using anti-solvent methods. Recrystallization is practiced for the purpose of purifying a material or for changing its particle size, or both. Generally describing current industrial recrystallization processes, a material to be recrystallized is dissolved in a liquid solvent, and a liquid anti-solvent, i.e., a non-solvent for the material, is added to the solvent. The solvent and anti-solvent are miscible liquids. At some concentration of anti-solvent in the solvent, the mixed liquid is no longer a solvent for the solid material, and precipitation of the solid occurs. Parameters such as the choice of solvent and anti-solvent, the initial concentration of solid in solution, the rate of admixing of the two liquids, and the direction of admixing, i.e., whether the anti-solvent is added to the solution or whether the solution is added to the anti-solvent, and other factors, combine to determine the particle size, particle size distribution, morphology, and other characteristics of the recrystallized material. Other parameters in the recrystallization process can include different temperatures for the two liquids so that thermal, as well as anti-solvent, considerations come into play in causing the solid to precipitate from solution.
The liquid solvent-liquid anti-solvent recrystallization process suffers to a lesser or greater degree in any particular application from several limitations. For example, it is sometimes difficult to attain extremely rapid and intimate admixing, and, thus, the concentration of solid is not uniformly changed throughout the liquid resulting in a wide particle size distribution. Additionally, liquid is frequently occluded in the recrystallized material or causes bubbles. During the drying process, the occluded material diffuses or vaporizes causing intragranular cavities which can affect the performance of the material in its use. On occasion, no matter what combination of parameters is used and no matter how well the effects of the parameters are understood, the desired particle characteristics are not achieved. For example, the explosive cyclotrimethylenetrinitramine (referred to as RDX in the explosives community) is recrystallized by liquid solvent--anti-solvent systems into 100 to 200.mu. particles that virtually always contain intragranular cavities as large as 50.mu.. A photomicrograph of this material is shown as FIG. 1.
As a practical matter, industrial applications require that the two liquids must be separated and reused. The only industrially viable separation process is distillation, and distillation of large volumes of liquid solutions. This is more so when the two components have similar boiling points, is energy intensive and thus, expensive. The ever-increasing cost of energy makes this a further reality.
There are other industrial processes for producing particles of a desired size. For example, simple mechanical comminution can be used to produce small particles in processes such as grinding or fluid energy milling using gases; the latter process employs impinging jets of air or nitrogen loaded with particles of material, collisions between particles resulting in their being comminuted. Fluid energy milling and grinding are difficult to apply to the problem of comminuting sensitive explosives because of the potential for explosions to occur. Pump grinding, i.e., recirculation of a slurry of particles through a pump, can more safely comminute sensitive materials, but the particles formed are usually very irregular. In the case of certain explosives, irregular, sharp-edged crystals confer extreme and undesired sensitivity to the material. For completeness, there are other industrial processes for forming crystals that are not recrystallization processes per se. For example, some materials are made by admixing solutions of two soluble species that, when the components contact, react or complex to form an insoluble and desired product or intermediate. This product can be used as is or it may subsequently dissolved in some other solvent and recrystallized by a liquid anti-solvent; the overall process of chemical reaction and recrystallization, of course, may suffer from the limitations described earlier if a liquid anti-solvent is used.